The present invention is directed, in general, to power electronics and, more specifically, to a bias supply circuit, method of operating a bias supply circuit and a switching power supply employing the circuit and method.
The electronics industry continues to develop smaller and more powerful equipment that perform many functions, which in turn may require increasingly more power from smaller, more compact power supplies. Additionally, such a power supply may also have internal bias supply requirements for additional voltages that are different from the main power supply voltages. Bias supplies are used in power supplies to provide operating voltages needed by internal power control circuits or other circuits that provide proper operation during start-up or sustained power supply operation. Bias supply voltages and power requirements typically add another level of complexity to an already demanding environment.
There are several present options for deriving a bias supply voltage such as that needed for control circuits that may be used in a small power supply unit. Typically, bias supply voltages of roughly 12 volts are often required. The voltage regulation accuracy requirement of such bias supplies is often in the neighborhood of ten percent. This tolerance requirement is more accurate than a simple unregulated approach will typically provide when based on the unregulated input voltage to the power supply.
A common approach, that is often not efficient, uses a dissipative regulator coupled to the input voltage source of the power supply. An alternative having better efficiency but requiring small but separate magnetics develops the bias voltage from an independent switching bias supply. A third approach uses a voltage developed in a winding coupled to an output inductor. This voltage may then be regulated further with a dissipative regulator. This particular approach possesses the disadvantage of needing to maintain safety separations of primary and secondary conductors wherein elements of the circuit are on both sides of the primary-secondary safety boundary of a power transformer.
A fourth approach may use a winding on the main power transformer that also employs a dissipative regulator. Other approaches using charge pumps, for example, also suffer from general inefficiencies. All of these approaches provide either poor efficiency or require numerous parts. Additionally, routing paths between the primary and secondary safety isolation regions of a printed wiring board wastes valuable board space in high density designs.
Accordingly, what is needed in the art is a bias supply circuit and method of providing a bias supply voltage that overcomes the deficiencies in the prior art. For instance, an approach that can be self-contained within the primary-side safety region of the power unit, that provides high efficiency without introducing new magnetic elements and does not require a separate regulation mechanism would be advantageous.
To address the above-discussed deficiencies of the prior art, the present invention provides a bias supply circuit for use with a switching power supply. The switching power supply employs a main switch and an auxiliary switch and has an input voltage and a voltage across a clamp capacitor coupled to a transformer. In one embodiment, the bias supply circuit includes a bias supply winding associated with the transformer and configured to provide a first voltage dependent on at least one of the input voltage and the voltage across the clamp capacitor during a conduction period of the main switch or the auxiliary switch. Additionally, the bias supply circuit further includes a bias supply storage capacitor coupled to the bias supply winding and configured to provide a second voltage dependent on at least another of the input voltage and the voltage across the clamp capacitor during a conduction period of another of the main switch or the auxiliary switch. A sum of the first voltage and the second voltage provides a bias supply voltage.
In another aspect, the present invention provides a method of operating a bias supply circuit for use with a switching power supply employing a main switch and an auxiliary switch and having an input voltage and a voltage across a clamp capacitor coupled to a transformer. The method includes providing a first voltage associated with a bias supply winding of the transformer and dependent upon at least one of the input voltage and the voltage across the clamp capacitor during a conduction period of the main switch or the auxiliary switch. The method also includes providing a second voltage associated with a bias supply storage capacitor coupled to the bias supply winding and dependent upon at least another of the input voltage and the voltage across the clamp capacitor during a conduction period of another of the main switch or the auxiliary switch. The method further includes forming a bias supply voltage by summing the first voltage and the second voltage.
The present invention also provides, in yet another aspect, a switching power supply including a primary power supply circuit that employs a main switch and an auxiliary switch and that has an input voltage and a voltage across a clamp capacitor coupled to a transformer. The switching power supply also includes a secondary power supply circuit, coupled to the transformer, that provides an output voltage. The switching power supply further includes a bias supply circuit, having a bias supply winding associated with the transformer that is configured to provide a first voltage that is dependent on at least one of the input voltage and the voltage across the clamp capacitor during a conduction period of the main switch or the auxiliary switch. The bias supply circuit also has a bias supply storage capacitor, coupled to the bias supply winding, that is configured to provide a second voltage dependent on at least another of the input voltage and the voltage across the clamp capacitor during a conduction period of another of the main switch or the auxiliary switch. A sum of the first voltage and the second voltage provides a bias supply voltage.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.